1. Field of Invention
The present invention relates to a focusing method, and more particularly to a focusing method for a digital camera for determining a focusing focal length that needs to be adjusted in an auto focusing stage.
2. Related Art
A user usually takes a picture with a digital camera in the following several stages. First, the digital camera is turned on; at this time, the digital camera enters a live view stage. Second, the user half presses a shutter after aiming at an object to be shot, and at the same time, the digital camera enters an auto focusing stage. Third, the user fully presses the shutter, and at this time, the digital camera enters a shooting stage and shoots the corresponding image picture.
The auto focusing stage refers to a focusing stage in which focusing is performed on the object to be shot and the shutter is not fully pressed for shooting. Briefly, it is a process in which the digital camera focuses on the object to be shot when the shutter is half pressed. During this process, a lens of the digital camera is moved to different focus positions (i.e., to different lens positions or steps, which are image sampling steps), and a degree of clarity or fuzziness of the image is determined according to a focus value acquired at each lens position.
Conventional focusing algorithms include a global search algorithm, a hill-climbing search algorithm, and a binary search algorithm. An effective search algorithm needs to consider the time required for search, the number of times for moving the lens, and search correctness. If too much search time is spent, the efficiency of auto focusing is lowered. If the lens is moved for too many times, additional battery power of the digital camera is consumed. On the contrary, the imaging quality of the digital camera is influenced.
For example, in the global search algorithm, a digital image acquired each time the lens is moved by one step is recorded; afterwards, a lens position corresponding to a digital image having the highest clarity is extracted; then, the lens is moved to the position corresponding to the highest clarity to achieve auto focusing, as shown in the diagram of a contrast value curve of FIG. 1.
Although the prior art can find out a focusing focal length for the object to be shot precisely, the digital camera still needs to refocus each time the digital camera performs focusing. In this way, the focusing time and power consumption of the digital camera are influenced.